Make cell_measure(...) work again.

Make cell_measure(...) work again.

A result of d8bafee27cc was that users can no longer write

cell_measure(...)

with no given template argument (in the codimension zero case). This
commit fixes that.

Note: since dim does not explicitly appear alone and unmodified in the
function arguments of cell_measure<dim, spacedim>, that function can
never have its type (more exactly, the value for dim) inferred correctly
from just the arguments. Put another way, one must always write
cell_measure<dim, spacedim> to use this function. This was not a problem
in the past when Point<dim> appeared instead of Point<spacedim>.